Integrated versus external transformer
Modern variable frequency drives offer two options: integrated transformer or external transformer. How do these options compare? Which one is a better choice?
INTEGRATED TRANSFORMER
EXTERNAL TRANSFORMER
There is no general answer. Instead, it depends on several aspects and project specific conditions. This post will walk you through the advantages and disadvantages of each solution.
Integrated transformer
Integrated transformer is a popular solution, especially in lower power range. Integrated means that the transformer is integral part of the VFD cubicle. The users like it for its simplicity. All the internal wiring between the transformer secondary windings and the rectifier section of the VFD is already done. The user just connects the 3-phase power to the primary terminals. Integrated transformer is mainly associated with multi pulse diode rectifier type of VFD. This is where the integrated version benefits from interconnection power cabling done in VFD factory.
Integrated transformers are dry type transformers. The insulation system is VPI (vacuum pressure impregnated). The transformer shall be as compact as possible since the dimensions impact the overall dimensions of the VFD. Therefore, integrated transformers are air cooled with forced ventilation. The required air flow depends on transformer rating. Generally, it is quite high volume of air that is needed.
The integrated solution comes with certain limitations and drawbacks as well. The input voltage is limited. Th maximum level is typically 11 kV or 13.8 kV. If the grid voltage is higher (e.g. 20 kV or 33 kV) then an additional step-down transformer would be required. The transformer is normally air cooled and the heat losses are evacuated into the electric room. It results into significant increase of size of HVAC and higher operational cost. The air flow inside the room is high, especially when multiple VFD units with integrated transformers are installed in the same room. The noise level is generally higher. The arc resistant design is more difficult to achieve. A short circuit on the primary side of the transformer (e.g. terminal field) will result in a very high fault current and consequently very high energy in case of arc fault. The integrated transformer requires more footprint inside the building. Integrated transformer is considered as part of VFD (internal component) and standardized. Engineered options are limited.
External transformer
External transformer is the first choice in installations with higher power or higher voltage. Both liquid filled or dry type transformer technologies are suitable. Dry type transformer is common up to voltage class of 36 kV. Higher voltage levels might be possible (depending on supplier), but are very rare. Liquid immersed VFD transformers cover the entire voltage range and there are several references with 132 kV primary voltage of VFD transformer. Direct connection to higher distribution voltage allows the elimination of one step-down transformer and its losses. Upstream grid also has higher short circuit capacity resulting in lower harmonic distortion. External transformer can be located in another building or outdoors. The transformer losses are dissipated into the outdoor ambient and no re-cooling is required. Cooling is very flexible – almost any kind of cooling can be used. The transformer is typically designed for a project and can be specifically optimized. External transformers can achieve higher efficiency compared to integrated transformers as they are usually less restricted in terms of dimensions.
Drawback of external transformer is the system engineering effort. Someone has to think about topics such as protection concept, cooling, cabling etc. As explained in previous post (HERE) the considerations for VFD duty transformer are different than for a distribution transformer. In case of a multi-winding transformer with long distance to the VFD the cost of power cables might be significant. For that reason the multi-cell VFD topology comes with integrated transformer only.
Remark:
Some manufacturers, e.g. ABB, offer a third solution as well. This is called a semi-integrated or combined transformer. The concept leverages on advantages of both solutions. It will be described in a separate post. Coming soon!
Integrated versus external transformer
Coming to the point – let’s look at several aspects of integrated versus external transformer:
1. Footprint / space requirements
At lower power range (hundreds of kW) the VFD with integrated transformer is just marginally larger than VFD with external transformer. It is due to the fact that the small transformer can be “squeezed” into VFD enclosure. For larger ratings the transformer usually requires its own cabinet and the impact on footprint is more significant. If the space inside the electric room is limited then the external transformer tends to be a better solution.
2. Losses and cooling
VFD itself has efficiency approx. 98.1 – 98.8% (depending on size). The transformer efficiency is very similar. In other words the VFD with integrated transformer dissipates roughly twice the amount of losses of VFD with external transformer (see also Table 1 and 2). In lower power range the additional transformer losses are not much of a concern. However, as the power increases the demand for re-cooling capacity raises as well. Large air conditioning is costly (both CapEx and OpEx). Evaluated over the service lifetime of the drive system the cooling has not negligible impact on the overall cost.
3. Efficiency
The total efficiency of VFD with integrated transformer and VFD with external transformer is in similar range. The integrated transformer is optimized for minimum physical dimensions and operates with high temperatures: class H (180°C) or even 220°C. It means that integrated transformer usually has somewhat higher losses than his external counterpart. The external transformer has less restrictions in terms of dimensions and can be designed for higher efficiency if requested.
4. Input voltage range
The integrated transformer version is suitable for voltage range 3 kV – 11 kV (13.8 kV). Higher voltages are usually not possible. The limitation comes from the restricted dimensions. Higher voltage increases the height of input transformer itself as well as the required clearances. Higher voltage level also means stronger grid with higher short circuit level → protection concern.
The external transformer is flexible with regards to input voltage. The only limitation is the capability of the manufacturer. There are references with primary voltage of VFD transformer as high as 132 kV. Such installations are of course very rare. 20 kV or 33 kV on the other hand are very common.
5. Power cables
VFD with integrated transformer comes with all internal power connections already installed. This is very practical for the user as it minimizes the installation effort as well as potential errors in cabling. In case of external transformer someone has to take care of cable dimensioning and laying. Multi-winding VFD transformers require multiple phases and corresponding amount of cables.
6. Protection
Transformer protection is essential. Integrated transformer comes with temperature monitoring and overvoltage protection. Overcurrent protection relay is often included as well. The alarm and trip limits are standardized and ‘field proven’ for 5-winding or 7-winding transformers. Exception are the transformers for multi-cell VFD topology with large quantity of secondary windings (often 9, 15 or even 21) – these are practically impossible to protect!
External transformer needs its dedicated protection. The protection elements depend on the transformer type. Dry type includes temperature monitoring, liquid filled typically Buchholz relay and pressure relief device. The surge arrestors are optional, but highly recommended. The protection settings are usually done project specifically, but guidelines are available.
A comparison of both transformer options is provided below. In order to make it more conclusive we have selected two power ratings:
– 1’500 kW motor power
– 5’000 kW motor power
The key parameters are listed in below tables:
Table 1: Integrated versus external transformer for 1’500 kW variable frequency drive system
Table 2: Integrated versus external transformer for 5’000 kW variable frequency drive system
Summary
What type of transformer should you select in your project? Well, every installation might have its unique aspects and considerations. Therefore, there is no general guideline. Based on above comparison you shall figure out what is the best for you. As very rough guideline we might say that integrated transformers are preferred in low power range where additional losses in the electric room do not create a big burden. As the power rating increases the external transformer becomes more interesting.
Our opinion
VFD with integrated transformer is an elegant solution in lower power range. The customer typically values the simplicity and ease of installation (3 cables in / 3 cables out). The footprint of VFD is not very much impacted due to the transformer (VFD with integrated transformer is just marginally bigger than VFD with external transformer – see Table 1 above). The basic transformer protection is already included as part of standard product. It saves certain engineering effort as the protection settings (alarm and trip levels) are already pre-defined by the manufacturer. However, transformer protection remains an issue for multi-cell VFD topology with up to 21 transformer secondary windings. The transformer losses at lower power can be handled by air conditioning system without too much impact on HVAC price and cost of ownership.
VFD with external transformer is preferred in higher power range. The transformer losses are not negligible anymore and would have significant impact on the size of air conditioning unit and operation cost. High power integrated transformer also significantly affect the overall VFD footprint. Therefore, it might be better to place the transformer outdoors. In case of higher grid voltage (> 13.8 kV) the external transformer is usually the only solution (additional step-down transformer would be associated with extra investment and additional losses). Large capacity VFDs are preferably connected to higher voltage level anyway (if available) to minimize the impact on the grid. The external transformer provides full flexibility in technology and cooling ranging from IP00 dry type transformer (indoor in naturally ventilated room) over IP55 AFAF or AFWF dry transformers up to liquid immersed transformers with mineral oil and synthetic or natural esters, respectively.
References
[1] ABB medium voltage drive portfolio, https://new.abb.com/drives/medium-voltage-ac-drives
[2] Multi-winding VFD transformer design, https://mb-drive-services.com/vfd_transformer_design/
[3] Combined transformer, https://mb-drive-services.com/combined-transformer/
